344 VIII. HEMATIN ENZYMES, I. CYTOCHROME SYSTEM 



3.2. Spectroscopic Observations on Cytochromes 



The first to observe the cytochromes spectroscopically was Mac- 

 Munn {1833,1836-1838). He called them histohematin, or, since he 

 also observed them in the muscle, myohematin. Premature attempts 

 at isolation from the cell led to a confusion with myohemoglobin 

 (1069,1725) and to criticism of the work by Hoppe-Seyler. Mac- 

 Munn's observations remained practically forgotten for more than 

 thirty years, when Keilin {1474.,14'^5) confirmed and greatly extended 

 them by careful microspectroscopic studies. Keilin's observations 

 were immediately confirmed by Schumm's work {21^98). In bakers' 

 yeast Keilin found the following four-banded absorption spectrum, 

 the fourth band having apparently three maxima: a, 604; b, 566; 

 c, 550; and d, 532, 528, and 521 my.. 



On vigorous aeration these bands disappeared, while on exclusion 

 from air or reduction they reappeared. If the respiratory ferment is 

 poisoned by cyanide, the cytochrome bands become strong; the 

 dehydrogenase systems of the cells in conjunction with other non- 

 hematin catalysts reduce the hemfchromes to hemochromes. The 

 disappearance of the bands on aeration is in fact no total disappear- 

 ance, but the hemichrome absorption bands are too faint and indis- 

 tinct to be seen in the usual manner in the microspectroscope. 

 Occasionally the hemichrome bands of ferricytochrome c (566.5 mju; 

 stronger, 529 \nn) may have been mistaken for hemochrome bands 

 of cytochromes b (c/. 2079, p. 150; 281^3). 



The close relationship of the spectrum given above to the two- 

 banded hemochrome spectrum is not immediately apparent. Keilin 

 correlated the bands in the following manner (1^76) : 



This correlation was later proved to be correct for cytochrome c 

 and probably for cytochrome b, although the /3 band of cytochrome 

 c itself displays a light cannelation (597,1477). That cytochrome a 

 has a second absorption band in the visible region is not clearly proved 

 (cf. Sect. 3.5.). The close relationship of the absorption spectrum of 

 ferrocytochrome c to that of hemochromes with saturated side chains 

 such as mesohemochrome, of ferrocytochrome b to that of a proto- 



